Waste heat is often created as a byproduct of industrial processes where flowing streams of high-temperature liquids, gases, or fluids must be exhausted into the environment or removed in some way in an effort to maintain the operating temperatures of the industrial process equipment. Some industrial processes utilize heat exchanger devices to capture and recycle waste heat back into the process via other process streams. However, the capturing and recycling of waste heat is generally infeasible by industrial processes that utilize high temperatures or have insufficient mass flow or other unfavorable conditions.
Waste heat can be converted into useful energy by a variety of turbine generator or heat engine systems that employ thermodynamic methods, such as Rankine cycles. Rankine and similar thermodynamic cycles are typically steam-based processes that recover and utilize waste heat to generate steam for driving a turbine, turbo, or other expander connected to an electric generator, a pump, or other device.
An organic Rankine cycle utilizes a lower boiling-point working fluid, instead of water, during a traditional Rankine cycle. Exemplary lower boiling-point working fluids include hydrocarbons, such as light hydrocarbons (e.g., propane or butane) and halogenated hydrocarbon, such as hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs) (e.g., R245fa). More recently, in view of issues such as thermal instability, toxicity, flammability, and production cost of the lower boiling-point working fluids, some thermodynamic cycles have been modified to circulate non-hydrocarbon working fluids, such as ammonia.
Generally, the pressure of the low-pressure side of a Rankine cycle, upstream from the system pump, must be controlled to protect expensive components within the heat engine system, to maintain desired pressures of the high-pressure side, and to maximize efficiencies of the heat engine. If the low-pressure side drops below the saturation point of the working fluid, cavitation can occur, which can damage the pump. On the other hand, the pressure ratio between the low-side and the high-side is directly related to the power generation of the system, with efficiency and power generation being highly sensitive to changes in the low-pressure side, even as compared to the high-pressure side.
Accordingly, it is desirable to maintain control of pressure in the low-pressure side. In the past, systems of vents, pressure containment vessels, and other equipment have been used as mass management systems, with a good degree of success, to maintain desired operation parameters. However, these systems often allow pressure to be vented to the system on nearly a constant basis. This represents wasted working fluid, which must be replenished on a periodic basis, thereby increasing operating costs.
Therefore, there is a need for a heat engine system, a mass management system, a method for regulating pressure in the heat engine system, and a method for generating electricity, whereby the systems and methods provide maintaining a fine control of the working fluid inventory within the system to have the desired range of pressure within the system, avoiding large pressure differentials in the low-pressure and high-pressure sides, avoiding ongoing ventilation of the process fluid, and maximizing the efficiency of the heat engine system to generate work or electricity.